Reflective optical element and euv lithography appliance

ABSTRACT

A reflective optical element and an EUV lithography appliance containing one such element are provided, the appliance displaying a low propensity to contamination. The reflective optical element has a protective layer system includes at least two layers. The optical characteristics of the protective layer system are between those of a spacer and an absorber, or correspond to those of a spacer. The selection of a material with the smallest possible imaginary part and a real part which is as close to 1 as possible in terms of the refractive index leads to a plateau-type reflectivity course according to the thickness of the protective layer system between two thicknesses d 1  and d 2 . The thickness of the protective layer system is selected in such a way that it is less than d 2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of application Ser. No. 12/399,775filed Mar. 6, 2009, which is a Continuation Application of applicationSer. No. 11/216,560 filed Aug. 31, 2005, which is a Continuation-in-PartApplication of PCT Application No. PCT/EP2004/002014 filed Mar. 1, 2004and published as WO 2004/079753 on Sep. 16, 2004, which claims priorityfrom German Application No. 103 09 084.3 filed Mar. 3, 2003. The entiredisclosures of the prior applications are hereby incorporated byreference.

FIELD OF THE INVENTION

The invention concerns a reflective optical element for the EUV and softX-ray wavelength region, having a multilayer system and a protectivelayer system, wherein the side of the multilayer system facing theprotective layer system terminates in an absorber layer. Furthermore,the invention concerns an EUV lithography appliance with a reflectiveoptical element of this kind.

BACKGROUND OF THE INVENTION

Multilayers are composed of periodic repetitions, and in the most simplecase a period consists of two layers. The one layer material shouldconsist of a so-called spacer material, while the other layer materialshould consist of a so-called absorber material. Spacer material has areal part of the refractive index close to 1, absorber material has areal part of the refractive index significantly different from 1. Theperiod thickness and the thicknesses of the individual layers are chosenin dependence on the operating wavelength, so that the reflectivity isgenerally maximized at this operating wavelength.

Depending on the requirement of the reflective optical element in regardto the reflection profile, various configurations of the multilayersystem are conceivable. Bandwidth and reflectivity, for example, can beadjusted by having more than just two materials in one period or bydeviating from a constant layer thickness or even from constantthickness ratios (so-called depth-graded multilayers).

EUV lithography appliances are used in the production of semiconductorcomponents, such as integrated circuits. Lithography appliances whichare used in the extreme ultraviolet wavelength region primarily havemultilayer systems of molybdenum and silicon, for example, as theoptical reflective element. Although EUV lithography appliances have avacuum or a residual gas atmosphere in their interior, it is notentirely possible to prevent hydrocarbons and/or other carbon compoundsfrom being inside the appliance. These carbon compounds are split apartby the extreme ultraviolet radiation or by secondary electrons;resulting in the depositing of a carbon-containing contamination film onthe optical elements. This contamination with carbon compounds leads tosubstantial reflection losses of the functional optical(surfaces, whichcan have a considerable influence on the economic effectiveness of theEUV lithography process. This effect is intensified in that typical EUVlithography appliances have eight or more reflective optical elements.Their transmission is proportional to the product of the reflectivitiesof the individual optical reflective elements.

The contamination leads not only to reflectivity losses, but also toimaging errors, which in the worst case make an imaging impossible.Thus, cleaning cycles have to be provided when operating an EUVlithography appliance or when using reflective optical elements. Thesesignificantly increase the operating costs. But the cleaning cycles notonly increase the down time, but also entail the risk of worsening ofthe homogeneity of the layer thickness of the reflective opticalelements and the risk of increasing the surface relief, which leads tofurther reflectivity losses.

One approach to controlling the contamination for Mo/Si multilayermirrors is found in M. Malinowski et al., Proceedings of SPIE Vol. 4688(2002), pages 442 to 453. A multilayer system of 40 pairs of molybdenumand silicon with pair thickness of 7 nm and a Γ=(d_(Mo)/(d_(Mo) d_(Si)),with d_(Mo) being the thickness of the molybdenum layer and d_(Si) thethickness of the silicon layer, of around 0.4, was provided with anadditional silicon layer on the uppermost molybdenum layer. Multilayersystems with different thickness of silicon protective layer weremeasured, extending from 2 to 7 nm. Traditional Mo/Si multilayer systemshave a silicon protective layer of 4.3 nm, which helps protect againstcontamination, although it very quickly becomes oxidized. Themeasurements revealed that there is a reflectivity plateau for a siliconprotective layer of 3 nm, depending on the radiation dose. It istherefore recommended to use silicon protective layers with a thicknessof 3 nm, instead of silicon protective layers with a thickness of 4.3nm. For a longer operating time can be achieved with a siliconprotective layer 3 nm in thickness, for the same tolerance in thereflectivity loss.

SUMMARY OF THE INVENTION

The problem of the present invention is to provide a reflective opticalelement for the

EUV and soft X-ray wavelength region that has the longest possiblelifetime. Furthermore, the problem of the invention is to provide an EUVlithography appliance with the shortest possible down time.

The problem is solved by a reflective optical element, as well as an EUVlithography appliance according to the claims.

It has been found that reflective optical elements for the EUV and softX-ray wavelength region with long lifetime are achieved if they areprovided with a protective layer system that has one or more layers ofmaterials with a particular refractive index, and in which the overallthickness of the protective layer system is chosen according toparticular criteria. The one or more layers of the protective layersystem should have a refractive index at operating wavelengths between12.5 nm and 15 nm whose real part is between 0.90 and 1.03, preferablybetween 0.95 and 1.02, and whose imaginary part is between 0 and 0.025,especially preferably between 0 and 0.015. Thus, as compared to thelayers of the multilayer system situated underneath, the layers of theprotective layer system have the optical properties of a spacer or liebetween those of a spacer and an absorber. The choice of a material withthe smallest, possible imaginary part and a real part as close aspossible to 1 results in a plateau-shaped reflectivity curve, dependingon the thickness of the protective layer system between two thicknessesd₁ and d₂. This means that, with these selected materials, thereflective optical element made of a multilayer system and a protectivelayer system is insensitive to fluctuations in the thickness of theprotective layer system in a particular region. According to theinvention, the reflective optical element has a protective layer systemwith a thickness smaller than d₂.

The reflective optical elements of the invention have the benefit thattheir relative insensitivity to thickness variations in the protectivelayer system also translates into an insensitivity to the build-up of acontamination layer. Without substantial change in reflectivity, muchthicker carbon layers can be tolerated than with traditional reflectiveoptical elements. This also has a positive impact on the homogeneity ofthe imaging, since even thickness fluctuations over the entire area arenegligible.

Basically, for a given operating wavelength, one will select thematerial, the layer makeup of the protective layer system, and theindividual layer thicknesses so that a plateau in the reflectivity isformed between two thicknesses d₁ and d₂ as a function of the thicknessof the protective layer system. The specific thickness of the protectivelayer system is then advantageously chosen to be as small as possible,but still within the reflectivity plateau. In practice, one must makesure that the minimum layer thickness is always observed for each layer,so that one can produce a closed layer.

It has been found that a standing wave field is formed by reflection atthe reflective optical element, whose minimum for a protective layerthickness d₁ lies in the vacuum at a distance of a fraction of theoperating wavelength. Now, if the layer thickness of the protectivelayer system is increased, the minimum of the standing wave fieldapproaches the surface. Accordingly, the value of the standing wavefield at the surface increases until the maximum is also achieved. Thus,the formation of the reflectivity plateau in dependence on the thicknessof the protective layer system results because, with increasing layerthickness, the additionally created absorption, i.e., the resultingdecrease in reflectivity, is compensated in that reflectivity gains areproduced by increasingly constructive interference after a certain layerthickness.

As an additional effect, fewer photoelectrons are emitted near theminimum of a standing wave field. Since the photoelectrons also breakdown the hydrocarbons from the residual gas atmosphere into carbon orcarbon-containing particles, this has the result of a noticeably slowerbuild-up of the contamination.

A preferred embodiment is therefore characterized in that the thicknessd₁ of the protective layer system is such that a standing wave formed byreflection at operating wavelength λ_(B) has a minimum at a distancefrom the surface of the reflective optical element of 0.4, or less.Thus, the minimum lies in the vacuum. With increasing thickness, thesurface as it were migrates through the minimum until the thickness d₂is reached. This corresponds to a distance from the surface to theminimum of at most 0.24, and the minimum is located inside thereflective optical element.

The specification of the essentially constant curve of the reflectivityis to be understood as meaning that all reflectivity fluctuations in aregion that does not limit the functional capability of the reflectiveoptical element are considered to be constant. In an especiallypreferred embodiment, a reflectivity decrease of 1% of the maximumreflectivity in the protective layer thickness region between d₁ and d₂is considered harmless and regarded as being a constant reflectivitycurve in the context of this invention.

It is to be assumed that the reflectivity curve as a function of theprotective layer thickness between d₁ and d₂ goes through at least oneinflection point at the protective layer thickness d_(w). For due to thepartial compensation of the reflectivity loss in the protective layerthickness region between d₁ and d₂, the slope of the reflectivity curvechanges in this protective layer thickness region. Advantageously, theparticular thickness of the protective layer system is chosen to be≦d_(w). This ensures that the thickness of the protective layercorresponds to a reflectivity which lies in the region of constantreflectivity in the sense of this invention. As a result, the reflectiveoptical element becomes insensitive to an increase in the thickness ofthe protective layer, for example, due to contamination.

In an especially preferred embodiment, the thickness of the protectivelayer system is equal to d₁.

The advantageous properties of the invented reflective optical elementhave especially positive impact when they are used in an EUV lithographyappliance Especially when several reflective optical elements areconnected in succession, the more uniform reflectivity and also moreuniform field illumination for lengthy periods of time have especiallypositive impact. It has been found that even with increasingcontamination the wavefront errors in the complex optical systems of EUVlithography appliances can be kept small. A major benefit consists inthat fewer cleaning cycles are required for the EUV lithographyappliance, thanks to the longer lifetime of the reflective opticalelements. This not only reduces the down time, but also the risk ofdegeneration of the layer homogeneity, greater roughness of the surface,or. partial destruction of the uppermost protective layer from toointense cleaning are significantly reduced. In particular, the cleaningprocesses for the reflective optical elements of the invention can becontrolled such that the contamination layer is deliberately notentirely removed, but rather a minimal contamination layer alwaysremains on the uppermost layer. This protects the reflective opticalelement against being destroyed by too intense cleaning. The thicknessof the contamination layer can be measured in traditional manner duringits build-up or during the cleaning with a suitable in situ monitoringsystem.

It has proven to be especially advantageous for the protective layersystem to consist of one or more materials from the group Ce, Be, SiO,SiC, SiO₂, Si₃N₄, C, Y, MoSi₂, B, Y₂O₃, MoS₂, B₄C, BN, Ru_(x)Si_(y), Zr,Nb, MoC, ZrO₂, Ru_(x)Mo_(y), Rh_(x)Mo_(y), and Rh_(x)Si_(y). The SiO₂should preferably be amorphous or polycrystalline.

The best results are achieved with a multilayer system that consists ofMo/Si layers and that ends with the molybdenum layer on the side facingthe protective layer system. Depending on the operating wavelength,multilayer system, and requirement for the reflective optical element,it can be advantageous for the protective layer system to consist ofprecisely two or precisely three layers.

In a preferred embodiment, the protective layer system terminates towardthe vacuum with a layer of a material for which the build-up ofcarbon-containing substances is suppressed. It has been found thatcertain materials have a low affinity for carbon-containing substances,in other words, carbon-containing substances get stuck to them only witha low probability or they have a slight adsorption rate. Thus, for thesematerials, the build-up of carbon-containing substances is drasticallyreduced or suppressed. It has been found that such materials can be usedas a protective layer for reflective optical elements for the EUV andsoft X-ray wavelength region, without showing significant negativeeffects on the optical behavior of the reflective optical element.Especially preferred as such are the materials ZrO₂, Y₂O₃, and silicondioxide in various stoichiometric relations. The silicon dioxide can bein the amorphous or polycrystalline, or possibly even the crystallinestate.

In another preferred embodiment, the protective layer system terminatestoward the vacuum with a layer of a material that is inert to energydeposition, that is, to bombardment with EUV protons or to externalelectric fields. This decreases the probability of spontaneous electronemission, which in turn might split apart the residual gases intoreactive cleavage products. Hence, the deposition of contamination onthe protective layer system is further reduced. One can influence theinertia to external electromagnetic fields, for example, by giving thesurface the lowest possible relief and/or using materials that have alarge gap between the valency band and the conduction band. Especiallypreferred for this are the materials Nb, BN, B₄C, Y, amorphous carbon,Si₃N₄, SiC, as well as silicon dioxide in various stoichiometricrelations. The silicon dioxide can be in the amorphous orpolycrystalline, or possibly even the crystalline state.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall be explained more closely by means of severalexamples and the figures. These show:

FIG. 1 shows the reflectivity of a multilayer with protective layersystem as a function of the thickness of the protective layer system;

FIGS. 2 a and 2 b show the position of the standing wave field fordifferent thicknesses of the protective layer system;

FIG. 3 shows the reflectivity of a first reflective optical element as afunction of the thickness of the contamination layer;

FIGS. 4 a and 4 b shows the dependency of the wavefront error on thethickness of the contamination layer for a six-mirror system for EUVlithography;

FIG. 5 shows the reflectivity of a second reflective optical element asa function of the thickness of the contamination layer; and

FIGS. 6 a and 6 b show the structure of a reflective optical elementincluding a multilayer system and a protective layer system according toexemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 6 a and 6 b show schematically the structure of exemplaryembodiments of a reflective optical element 10 for the EUV and softX-ray wavelength region comprising a multilayer system 12 and aprotective layer system 13, 13′ on a substrate 11. The multilayer system12 is made of spacer layers 14 and absorber layers 15. The multilayersystem 12 terminates with an absorber layer 15 on which the protectivelayer system 13, 13′ is arranged. In exemplary embodiments, themultilayer system 12 consists of silicon layers as spacer layers 14 andof molybdenum layers as absorber layers 15.

The protective layer system 13, 13′ has at least one layer of a materialwith a refractive index whose real part at an operating wavelengthbetween 12.5 nm and 15 nm is between 0.90 and 1.03, preferably between0.95 and 1.02, and whose imaginary part at an operating wavelengthbetween 12.5 nm and 15 nm is between 0 and 0.025, preferably between 0and 0.015, so that the reflectivity plotted as a function of thethickness of the protective layer system 13, 13′ at first drops, until athickness d₁ is reached, the reflectivity remains essentially constantbetween thickness d₁ and another thickness, d₂, where d₂>d₁, and thereflectivity further drops for a thickness>d₂, and the thickness of theprotective layer system 13, 13′ is smaller than d₂. The protective layersystem 13, 13′ consists of one or more materials from the group Ce, Be,SiO, SiC, SiO₂, Si₃N₄, C, Y, MoSi₂, B, Y₂O₃, MoS₂, B₄C, BN,Ru_(x)Si_(y), Zr, Nb, MoC, ZrO₂, Ru_(x)Mo_(y), Rh_(x)Mo_(y), orRh_(x)Si_(y).

In exemplary embodiments of the present invention, the protective layersystem 13, 13′ consists of two layers 16, 17 (see FIG. 6 a) or threelayers 16′, 17′, 18 (see FIG. 6 b). Advantageously, the protective layersystem 13 of the example illustrated in FIG. 6 a ends on a side of avacuum, i.e. terminates with a layer 16 of a material for which thebuild-up of carbon is suppressed, and the protective layer system 13′ ofthe example illustrated in FIG. 6 b ends toward a side of a vacuum, i.e.terminates with a layer 17′ of a material that is inert to energydeposition. It will be noted that the terminating of the protectivelayer system 13, 13′ with a layer of a material for which the build-upof carbon is suppressed or with a layer of a material that is inert toenergy deposition or with an other kind of layer is independent of theprotective layer system consisting of one, two, three or more layers, aswell as independent of the features of any other layers present in theprotective layer system.

Example 1

On a Mo/Si multilayer system located on a substrate of amorphous silicondioxide, consisting of 50 pairs of 2.76 nm molybdenum and 4.14 nmamorphous silicon (a-SiO₂), a three-layer protective layer system isdeposited. The protective layer system borders on the uppermostmolybdenum layer of the multilayer system with a Y-layer 1.2 nm thick.On the Y-layer is placed a 1.5 nm Y₂O₃ layer. At the vacuum side, theprotective layer system is closed by a 1 nm thick amorphous silicondioxide layer. The choice of the materials and their thickness is basedon the criteria of the invention. In particular, the materials are alsoselected so as to suppress carbon build-up (Y₂O₃, a-SiO₂) or to be inertto energy deposition (Y, a-SiO₂).

Disregarding the interface and surface roughness, one obtains areflectivity of 70.2% at an operating wavelength of 13.5 nm for an angleof incidence of 0° with the normal to the surface. FIG. 1 shows thereflectivity of the entire reflective optical element under theseconditions as a function of the thickness of the protective layersystem, but holding constant the thickness of 1.2 nm for Y and 1.5 nmfor Y₂O₃. A distinct reflectivity plateau is formed between a thicknessd₁=3.7 nm and a thickness d₂=6.68 nm of the protective layer system, oran a-SiO₂ layer of 1 nm and 2.98 nm. Accordingly, the thickness of thesilicon dioxide layer was selected to be 1.0 nm.

In FIGS. 2 a and 2 b, the resulting standing wave field is shown for aprotective layer system thickness of 3.7 nm (FIG. 2 a) and for aprotective layer system thickness of 6.68 nm (FIG. 2 b). Segments a-ccorresponding to the protective layer system of amorphous SiO₂ (a), Y₂O₃(b), and Y (c) and segments d, e corresponding to the multilayer systemof molybdenum (d) and amorphous silicon (e). As can be clearly seen,with increasing thickness of the protective layer system the surface ofthe reflective optical element is situated in the vicinity of theminimum of the existing wave field, it migrates through the minimum, soto speak. This would suggest a slight contamination due to secondaryelectrons.

FIG. 3 shows the reflectivity of the reflective optical element with theprotective layer system of Y, Y₂O₃, and a-SiO₂ as a function of thebuilt-up contamination layer. If one selects a tolerance range of 1% forthe fluctuation in reflectivity, a carbon layer up to 4 nm thick can betolerated without significant change in reflectivity. The operating timeis therefore a multiple higher than for traditional reflective opticalelements.

In FIG. 4 a and b, these positive results are also shown by means of anEUV lithography appliance with six reflective optical elements (S1-S6)according to the invention as mirrors. The tested mirror construction isshown in FIG. 4 a. In FIG. 4 b, the wavefront error is shown as afunction of the carbon thickness.

Although the wavefront varies periodically with the increased carbonthickness, the absolute value of the wavefront error does not exceed avalue that would significantly impair the imaging quality of thelithography system for any carbon thickness.

Because of the insensitivity of the reflective optical element discussedhere with respect to the build-up of a carbon contamination layer, it ispossible to only remove the contamination layer down to a layer of 0.5nm when cleaning the reflective optical element or when cleaning theentire EUV lithography appliance. This will ensure, on the one hand,that the cleaned optical element once again has a long lifetime. But itwill also make sure that the risk of degeneration of the layerhomogeneity or roughening of the surface or partial destruction of thetopmost layer by too intense cleaning is reduced.

Example 2

On a multilayer system of 50 Mo/Si pairs located on an amorphous silicondioxide substrate, optimized for an operating wavelength of 13.5 nm, aprotective layer system of a 2.0 nm thick cerium layer, which adjoinsthe topmost molybdenum layer of the multilayer system, and a 1.5 nmthick silicon dioxide layer is placed. The minimum of a standing waveproduced by reflection on the uncontaminated reflective optical elementat operating wavelength A_(B) lies in the vacuum, 0.05 λ_(B) from itssurface. For a maximum reflectivity of 70.9% at an operating wavelengthof 13.5 nm and a tolerated reflectivity decrease of 1%, a carboncontamination layer can tolerate a thickness of up to 3.5 nm (see FIG.5). This reflective optical element as well is suitable for use in anEUV lithography appliance.

The above description of the preferred embodiments has been given by wayof example.

From the disclosure given, those skilled in the art will not onlyunderstand the present invention and its attendant advantages, but willalso find apparent various changes and modifications to the structuresdisclosed. The applicant seeks, therefore, to cover all such changes andmodifications as fall within the spirit and scope of the invention, asdefined by the appended claims, and equivalents thereof.

1. A reflective optical element for the extreme ultraviolet (EUV) and soft X-ray wavelength regions, the reflective optical element comprising a multilayer system and a protective layer system, wherein: the protective layer system comprises at least two layers; the protective layer system terminates at a vacuum side in least one layer of a material with a refractive index whose real part at an operating wavelength between 12.5 nm and 15 nm is between 0.90 and 1.03, and whose imaginary part at an operating wavelength between 12.5 nm and 15 nm is between 0 and 0.025; and a first layer of the at least two layers of the protective layer system comprises one or more materials selected from the group consisting of SiO₂, Y₂O₃ and ZrO₂, while a second layer of the at least two layers of the protective layer system comprises one or more materials selected from the group consisting of silicon dioxide in various stoichiometric relations, Y and ZrO.
 2. The reflective optical element according to claim 1, wherein the first layer of the protective layer system comprises SiO₂, while the second layer of the protective layer system comprises silicon dioxide in various stoichiometric relations.
 3. The reflective optical element according to claim 1, wherein the first layer of the protective layer system comprises SiO₂, while the second layer of the protective layer system comprises SiO.
 4. The reflective optical element according to claim 1, wherein the first layer of the protective layer system comprises Y₂O₃, while the second layer of the protective layer system comprises Y.
 5. The reflective optical element according to claim 1, wherein the first layer of the protective layer system comprises ZrO₂, while the second layer of the protective layer system comprises ZrO.
 6. The reflective optical element according to claim 1, wherein the thickness of the protective layer system is equal to d₁.
 7. An extreme-ultraviolet (EUV) lithography appliance comprising at least one reflective optical element according to claim
 1. 8. A reflective optical element for the extreme ultraviolet and soft X-ray wavelength regions, the reflective optical element comprising a multilayer system and a protective layer system, wherein: a side of the multilayer system facing the protective layer system terminates in an absorber layer, the protective layer system comprises at least two layers, the protective layer system terminates at a vacuum side in one layer of a material with a refractive index whose real part at an operating wavelength between 12.5 nm and 15 nm is between 0.90 and 1.03, and whose imaginary part at an operating wavelength between 12.5 nm and 15 nm is between 0 and 0.025, the reflectivity plotted as a function of a thickness of the protective layer system by varying a thickness of the layer terminating the protective system at the vacuum side at first drops, until a first thickness value d₁ is reached, the reflectivity remains essentially constant between the first thickness value d₁ and a second thickness value d₂, where d₂>d₁, and the reflectivity further drops for a third thickness value>d₂, the thickness of the protective layer system is smaller than d₂, and the protective layer system comprises one or more materials selected from the group consisting of Ce, Be, SiC, Si₃N₄, Y, MoSi₂, B, Y₂O₃, MoS₂, Ru_(x)Si_(y), Zr, Nb, ZrO₂, Ru_(x)Mo_(y), Rh_(x)Mo_(y), and Rh_(x)Si_(y).
 9. The reflective optical element according to claim 8, wherein the imaginary part is between 0 and 0.015 and the real part is between 0.95 and 1.02.
 10. The reflective optical element according to claim 8, wherein the protective layer system terminates toward a vacuum with a layer of a material for which build-up of carbon is suppressed.
 11. The reflective optical element according to claim 8, wherein the protective layer system terminates toward a vacuum with a layer of a material that is inert to energy deposition.
 12. The reflective optical element according to claim 8, wherein the thickness of the protective layer system is such that a standing wave formed by reflection at an operating wavelength λ_(B) has a minimum at a distance from the surface of the reflective optical element of 0.1 λ_(B) or less.
 13. The reflective optical element according to claim 12, wherein the minimum lies in the vacuum.
 14. The reflective optical element according to claim 8, wherein the reflectivity curve as a function of the thickness of the protective layer system is constant in the context of a reflectivity decrease of 1% of the maximum reflectivity in the protective layer thickness region between d₁ and d₂.
 15. The reflective optical element according to claim 8, wherein the reflectivity curve as a function of the thickness of the protective layer system between d₁ and d₂ goes through at least one inflection point at the protective layer thickness d_(w) and the thickness of the protective layer system is less than d_(w).
 16. A reflective optical element for the extreme ultraviolet (EUV) and soft X-ray wavelength regions, the reflective optical element comprising a multilayer system and a protective layer system, wherein: a side of the multilayer system facing the protective layer system terminates in an absorber layer, and a reflectivity of the reflective optical element as a function of a thickness of a built-up contamination layer is constant within a tolerance range of 1% for the fluctuation in reflectivity up to a given contamination layer thickness that is no more than 10% larger than a thickness of the protective layer system.
 17. The reflective optical element according to claim 16, wherein the given contamination layer thickness is approximately equal to or less than the thickness of the protective layer system.
 18. The reflective optical element according to claim 16, wherein the protective layer system consists of two layers.
 19. The reflective optical element according to claim 16, wherein the protective layer system consists of three layers.
 20. The reflective optical element according to claim 16, wherein the protective layer system comprises one or more materials selected from the group consisting of Ce, Be, SiO, SiC, SiO₂, Si₃N₄, C, Y, MoSi₂, B, Y₂O₃, MoS₂, B₄C, BN, Ru_(x)Si_(y), Zr, Nb, MoC, ZrO₂, Ru_(x)Mo_(y), Rh_(x)Mo_(y), and Rh_(x)Si_(y).
 21. The reflective optical element according to claim 16, wherein the multilayer system is a system that consists of molybdenum and silicon layers, ending with a molybdenum layer on the side facing the protective layer system.
 22. The reflective optical element according to claim 16, wherein the protective layer system terminates toward a vacuum with a layer of a material for which the build-up of carbon is suppressed.
 23. The reflective optical element according to claim 16, wherein the protective layer system termiantes toward a vacuum with a layer of a material that is inert to energy deposition.
 24. The reflective optical element according to claim 16, wherein the thickness d₁ of the protective layer system is such that a standing wave formed by reflection at an operating wavelength λ_(B) has a minimum at a distance from the surface of the reflective optical element of 0.1 λ_(B) or less.
 25. An extreme ultraviolet (EUV) lithography appliance with at least one reflective optical element according to claim
 16. 26. A reflective optical element for the extreme ultraviolet (EUV) and soft X-ray wavelength regions, comprising: a multilayer system, a protective layer system; and a contamination layer that changes in thickness over time, wherein the thickness over time of the contamination layer produces fluctuations in reflectivity of the reflective optical element within a tolerance range of no more than 1%. 